61 
Further, if half a dozen separate estimations are made 
in both broths with the same sample, it is: most unusual to 
obtain any difference in result between any of the six. We 
have never discovered any fallacies in this method of 
estimating the number of any class of organisms. 
One question is still left to be discussed, viz., in most 
samples, is it worth while to go to the trouble of finding out the 
acid and gas line in glucose and lactose broth? The answer 
to this appears to be that in cases where very accurate know- 
ledge of the condition of the water at any particular time is 
desired, it is worth carrying out this test, troublesome though 
it is. For instance (this case actually occurred), in a 
raw river water a sample taken at the intake of a town supply 
showed that the acid and gas line in both broths was very 
close together, as compared with a sample from the middle of 
the river. Yet the number of glucose fermenters was about 
the same in the two samples. Inspection showed that the 
bank just above the intake was badly fouled by mill coolies. 
Again, if outside pollution finds its way into a filtered drink- 
ing water containing a large number of glucose fermenters, 
but few lactose fermenters, a rapid drawing together of the 
lines would be very strong evidence of pollution, though 
the increase in total number of glucose fermenters might be 
overlooked. Of course in all cases other evidence of pollution 
would probably be available, if thorough investigation was 
made. 
In all cases of raw water where there is no doubt as to 
the fact that pollution is present, it is often particularly de- 
sirable to know whether the pollution is recent or old standing ; 
or in other words, does the pollution still exist in the true 
feecal form or has it been much altered by natural forces ? In all 
such instances, it would be well worth while to put the sample 
through both glucose and lactose broth. The following 
conclusions are justified from the above results : 
(7) that when dealing with natural waters the acid and 
gas line in glucose broth is usually higher than 
in lactose. 
